1. Field of the Invention
The present invention generally relates to switched-capacitor circuits, and more particularly, to a pseudo-differential switched-capacitor circuit using an integrator-based common-mode stabilization technique.
2. Description of the Prior Art
Switched-capacitor circuits are electronic circuit elements generally used for signal processing, which work by moving charges into and out of capacitors when switches are opened and closed. Usually, non-overlapping signals are used to control the switches. Compared to the combination of resistors and capacitors commonly utilized in electronic circuits, the switched-capacitor circuits have more advantages since they determine the characteristics of the electronic circuits according to the ratio of the capacitances rather than the individual values of resistors and capacitors. Also, the switched-capacitor circuit has a smaller area on a silicon chip than a combination of capacitors and resistors. In view of this, switched-capacitor circuits are much more suitable for use within integrated circuits than the combination of capacitors and resistors.
Switched-capacitor circuits are typically used with amplifiers, such as fully-differential amplifiers or pseudo-differential amplifiers. A switched-capacitor circuit using a fully-differential amplifier is well known to have the advantages of high noise-immunity and signal swing. The fully-differential amplifier usually includes a tail current source, however, which disadvantageously limits the output signal range, and thus is unfavorable for low voltage design. In order to enlarge the output signal range, a pseudo-differential amplifier may need to be used in the switched capacitor circuits for low voltage design. The pseudo-differential amplifier, however, lacks the tail current source, making it weakly immune to common-mode noise, such as input common mode voltage disturbance. In this regard, common-mode feedback or other noise cancellation techniques are necessary for the switched capacitor circuit using the pseudo-differential amplifier.
U.S. Pat. No. 7,724,063 entitled “Integrator-based common-mode stabilization technique for pseudo-differential switched-capacitor circuits”, which is owned by the same assignee and inventor of the present application, discloses a pseudo-differential switched-capacitor circuit for overcoming common mode voltage disturbance. The disclosure is hereby incorporated by reference in its entirety. In this patent, a differential floating sampling scheme (DFS) is adopted to decrease the common-mode gain of the pseudo-differential amplifier, thereby restraining the effect of the input common mode voltage disturbance on the switched-capacitor circuits. In addition, an integrator is adopted to form a negative feedback loop to cancel the charge injection effect due to the switches. Therefore, this patent not only overcomes the input common mode voltage disturbance, but also overcomes the charge injection effect.
As, however, the feedback mechanism of the above-mentioned patent utilizes an integrator, which is innately weak to the sudden change of the input common mode voltage disturbance, the input common mode voltage disturbance of high frequency cannot be properly handled.